250 CHEMICAL FERTILIZERS 



in the process is enough to maintain the temperature. The 

 vapours pass to a condenser and scrubber down which a 

 little water flows. The liquid passing away from this 

 condenser is formalin. The gases pass to a condenser, where 

 the excess of methyl alcohol is recovered, and then on to a 

 scrubber where water washes the remaining methyl alcohol 

 out of the waste gases. The recovered alcohol goes into 

 circulation once more. The efficiency is only about 50-60 %. 



Formalin is diluted with from 100-10,000 times its 

 volume of water before use. Strengths varying between 

 i in 1000 and i in 10,000 have been used to destroy fungoid 

 spores on grain without reducing the germinating power of 

 the seed. 



Cyanides. Prussic acid is used for fumigating fruit 

 trees and greenhouse plants. Many insect pests can be 

 eradicated by this means, but much caution is needed to 

 prevent accident. The prussic acid is manufactured on the 

 spot as vapour by acting on potassium or sodium cyanide 

 with either sulphuric or phosphoric acid. Fairly pure 

 sodium cyanide is sometimes sold as 120 % potassium 

 cyanide, that being the equivalent values of the two salts. 

 When practicable the cyanide should be placed in a fairly 

 open vessel, the doors of the greenhouse closed, and the acid 

 admitted by a tube from outside. Failing the necessary 

 facilities for such treatment, a weaker acid such as phosphoric 

 acid is employed, when the operator has more time to get 

 away. A better safeguard is to enclose the cyanide in zinc 

 capsules, which dissolve slowly in the sulphuric acid and 

 allow time for closing the doors. In the U.S.A. a tent is 

 placed over a fruit tree and a basin of cyanide set on the 

 ground. It is not then difficult to add the acid and close 

 the tent quickly. In mushroom cellars the addition of the 

 acid from outside can usually be arranged for. A general 

 adoption of a " carton " system would make this method of 

 fumigation more practical. Correct quantities of cyanide 

 in thin zinc cases, accompanied by the correct amount of 

 acid in glass bottles, would enable the horticulturist to 

 exterminate insects with confidence. 



